1. Field of the Invention
The present invention relates to an engine exhaust gas purifying system.
2. Description of Related Art
As a system for purifying exhaust gas of a car engine, there is a catalytic converter system for purifying HC, CO, NOx and the like in exhaust gas by an oxidation reaction or an oxidation-reduction reaction, in which a carrier carrying noble metal such as platinum and rhodium as a catalyst is provided within an exhaust pipe.
In the catalytic converter system, the catalyst needs to be heated up to its activation temperature, e.g. 300.degree. C. to 400.degree. C. or more, to purify exhaust gas. However, it has had a problem that because it adopts a method of heating the catalyst by exhaust gas in general, the catalyst does not reach the activation temperature and almost no exhaust gas is purified right after the engine is started.
Then, it has been arranged so that a catalyst reaches the activation temperature as soon as possible by heat of exhaust gas by disposing the catalyst in the vicinity of the engine, by changing the carrier carrying the catalyst from ceramic to metal having better thermal conductivity or by forcibly heating the catalyst by a heater provided in the carrier.
Meanwhile, due to the tightening of the so-called regulation of exhaust gas, it is assumed to be required to increase the rate of purification over the harmful gas components such as HC, CO and NOx further for the future. In order to achieve that, it is necessary to purify exhaust gas from right after the start of the engine, for example, by disposing the catalyst at position closer to the engine. However, if the catalyst is put closer to the engine, the catalyst is exposed to hot exhaust gas. Then, there arises a problem that the purification rate drops in contrary because the catalyst deteriorates quickly under hot conditions in general.
In order to eliminate such a problem, there has been proposed an arrangement, as disclosed in Japanese Patent Application Laid-Open No. H. 6-93844 for example, in which an exhaust pipe is ramified near the engine so as to create a main passage and a bypass passage, an adsorbing member is disposed in the bypass passage and a main catalyst is disposed at the downstream side of a region where the bypass passage joins again with the main passage. A change-over valve (opening/closing mechanism) is provided at the region near the engine where the exhaust pipe is ramified. When a temperature of exhaust gas is low right after the start of the engine for example, the exhaust gas is caused to flow through the bypass passage by closing the main passage and by opening the bypass passage. As a result, the harmful components, or HC in particular is adsorbed by the adsorbing member when the temperature of the catalyst is low. When the engine is warmed up and combustion therein is stabilized, the change-over valve is switched so that the exhaust gas flows through the main passage. The low temperature HC remains adsorbed by the adsorbing member until the catalyst reaches its activation temperature. Then, when the exhaust gas becomes hot and the main catalyst is warmed up fully by the hot exhaust gas, the bypass passage is opened slightly by manipulating the change-over valve. As a result, the exhaust gas flows through the bypass passage and thereby the low temperature HC held by the adsorbing member is thermally desorbed. The desorbed HC is purified by the main catalyst disposed on the downstream side.
However, because the adsorbing member does not have enough heat resistance, it deteriorates when exposed to the hot exhaust gas for a long period of time. Further, pressure loss of the engine increases when exhaust gas is caused to flow through the adsorbing member. Due to that, even when the HC is desorbed below the heat resistant temperature of the adsorbing member, the drivability and fuel consumption drop during that time. Still more, because an amount of oxygen enough for purifying the desorbed HC, in addition to the harmful components which are emitted from the engine, is required, an air-fuel ratio must be slid to the lean side from the stoichiometric ratio. However, the rate of purification over NOx drops in engine operation shifted to the lean side. Accordingly, a period of the desorption process must be accurate and be shortened as much as possible.
Due to that, physical quantities such as temperature of an input/output section of the adsorbing member, temperature of the main catalyst, the air-fuel ratio and the like must be measured accurately in the desorption process performed by the exhaust gas purifying system described above and various sensors are added for that end. As a result, it has had problems that its control logic is complicated and the cost of the system increases.